#include<stdio.h>
main()
{
	int i=1;
	double t,sum=0;
	t=1.0/(3*i-2);
	while (t>=10e-5)
	{
		if (i%2==0)
			sum-=t;
		else
			sum+=t;
		i++;
		t=1.0/(3*i-2);
	}
	printf("sum=%lf\n",sum);
}